A recent significant advance in the art of firing a furnace has been the development of an apparatus and a process which allow one to employ oxygen or oxygen-enriched air as the oxidant gas in the furnace firing operation while substantially avoiding such heretofore unavoidable problems as burner and furnace damage due to high flame temperature, and poor furnace gas mixing due to the smaller volumes of oxidant and fuel gas employed. The above-mentioned apparatus and process is the subject of U.S. patent application Ser. No. 138,759, filed on Apr. 10, 1980, by J. E. Anderson, and is incorporated herein in its entirety by reference. Among the many advantages of the Anderson apparatus and process is the considerable reduction in nitrogen oxides emissions (herein also referred to as NOx) which are considered significant pollutants.
An often-employed method of firing a furnace is one which uses alternating high firing rates and low firing rates. By firing rate, is meant the rate at which fuel and oxidant is supplied to a burner or furnace and is expressed in units of heat per unit of time. Alternating high and low firing rate periods are employed as a convenient technique for maintaining the furnace temperature within a given range.
Generally, a furnace will have, for any particular application, a temperature at which it is intended to operate. One way of maintaining this intended temperature is to use the afore-mentioned high and low firing rate cycle. In practice, the furnace may be fired until it attains the intended temperature; the furnace is then fired again at the low firing rate until the furnace temperature drops to a predetermined minimum; the furnace is then fired at the high firing rate until the furnace temperature reaches a predetermined maximum, which is higher than the intended temperature. The furnace is then fired at the low firing rate until the predetermined minimum temperature is reached. The cycle continues as long as required by the application.
When a furnace is fired using oxygen or oxygen-enriched air as the oxidant and in such a manner as to cause aspiration of the surrounding furnace gases into the oxidant prior to combustion, as with the process described in said U.S. application Ser. No. 138,759, a marked increase in the level of NOx emissions may be observed when the high and low firing rate cycle is used to maintain an intended temperature, as compared to the level of NOx emissions found when the furnace is fired at an essentially constant firing rate.
Accordingly, it is an object of this invention to provide a process for firing a furnace which allows one to use oxygen or an oxygen-enriched gas as the oxidant and to use an alternating high firing and low firing rate cycle, while reducing the level of nitrogen oxides generated to an acceptable level.